This invention relates to stepping motors of the harmonic drive type. More particularly the invention is concerned with providing an improved flexspline or flexrotor structure for such motors whereby stepping at speeds considerably higher than hitherto attained can be accomplished while retaining an ability to deliver useful torque output at such speed.
Harmonic drive transmission devices, as initially set forth in U.S. Pat. No. 2,906,143 issued Sept. 29, 1959 to Musser, commonly comprise a circular ring gear; a coaxial, radially deflectible gear of slightly different diameter (hereinafter termed a flexspline) having teeth or splines cooperative with those of the ring gear; and a wave generating means (or "strain inducer") for circumferentially propagating a deflection wave in the flexspline. By reason of tooth or spline interengagement at spaced circumferential localities resultant from the flexspline radial deflection and a differential in the numbers of teeth on the gears, their relative rotation is effected.
When the wave generating means employs an electromagnetic field to induce the deflection, for instance as disclosed in U.S. Pat. Nos. 3,169,201 to Spring et al, and 3,331,974 to Proctor, a high-response, low inertia stepping motor is provided. Radial deflection force of the field is converted to reversible, rotary output in small steps of high precision. Driving movement of such stepping motors is directly proportional to the number of pulses in a train appropriately supplied. In addition to exhibiting high acceleration and deceleration rates, motors of this type have proved advantageous in other characteristics such as freedom from backlash, low "overshoot", and short "settling time". One high speed controller arrangement is disclosed, for example, in U.S. Pat. No. 3,402,334 issued to George C. Newton, Jr.; another illustrative driving-circuit having means for modifying a train of pulses to stepping motors of the type above mentioned is described, for instance, in U.S. Pat. No. 3,869,656 to Daniel Kennedy and Walter R. Woodward.
Inertia of a stepping motor of the type being described is low since only the deflected shape of the flexspline element of the actuator is moved (excluding the magnetic field itself). The teeth of the flexspline have been commonly formed on the exterior periphery of the very thin-walled tubes of stainless steel. As set forth, for instance, in the mentioned Proctor pat. 3,331,974 and in U.S. Pat. 3,496,395 to Newell, it has been customary to improve magnetic properties of such flexsplines or flexrotors by incorporating a wound bridging strip or concentric tubes of thin steel. U.S. Pat. No. 3,609,423 to Spring and Woodward further discloses variant forms of such a bridging coil or concentric inner, welded tubes for reducing rotor reluctance, a range of about seven convolutions of magnetic iron each having a thickness of 0.004 inch to 5 convolutions of 0.006 inch thickness each being hitherto deemed an optimum.
It has been continuously appreciated, of course that a flexspline should be machineable, elastic, and capable of fully recovering its shape upon removal of the deforming force. It has also been realized that, in accordance with ring theory, stiffness in a flexspline varies directly as the cube of its thickness, or the sum of the cubes of the thicknesses of the flexspline and turns of its bridging material. Long experience in the design, testing and usage of flexsplines for electromagnetic embodiments of harmonic drives indicated that such inductive metals as 17-4 PH stainless steel would perform satisfactorily and it was thought that utilizing stiffer flexsplines than provided by stock of about a maximum of 0.006 inches would provide excessive resistance to deflection to the extent that a stepping motor would become inoperative or commercially unacceptable because incapable of producing adequate output torque. Higher stepping rates were therefore hitherto generally believed to be unattainable with practical usage, and that any relatively higher rates would be "traded" for unacceptable shorter operating life.